In the production of such fibers, especially those used in telecommunication systems which are to have a selected radial refractive-index profile, it is known to provide a cylindrical preform or parison conforming to the selected index profile and then to draw the preform at an elevated temperature into a fiber of considerably smaller diameter. In a first step (a) of making the preform, a silicon halide is reacted with an oxidant in the presence of a doping agent to produce a resulting vitrifiable material which, in a second step (b), is deposited on a cylindrical supporting surface to form a porous tubular structure wherein the desired refractive-index profile is established by suitable dosing of the constituents; in a third step (c), this tubular structure is then collapsed at high temperature into a solid rod which thereupon is thermally vitrified in a fourth step (d) unless such vitrification has already occurred at an earlier stage.
Conventionally, the constituents used in step (a) are interacted in a vapor phase at correspondingly high temperatures. Step (b), therefore, utilizes the technique known as CVD (chemical-vapor deposition) which can be practiced on an inner surface of a supporting tube (ICVD) or on an outer surface of a supporting mandrel (OCVD). In both instances the chemical reactants are oxidized during deposition by the flame of a burner in an oxygen-enriched atmosphere.
The deposits thus formed on an inner or outer supporting surface consists essentially of powdered silica doped with the oxide or oxides of one or more reactants initially admixed with the vaporized silicon halide. Typical chemical reactions include the following: EQU SiCl.sub.4 +O.sub.2 .fwdarw.SiO.sub.2 +2Cl.sub.2 EQU 2BCl.sub.3 +(3/2)O.sub.2 .fwdarw.B.sub.2 O.sub.3 +3Cl.sub.2 EQU 2POCl.sub.3 +(3/2)O.sub.2 .fwdarw.P.sub.2 O.sub.5 +3Cl.sub.2 EQU GeCl.sub.4 +O.sub.2 .fwdarw.GeO.sub.2 +2Cl.sub.2
Generally, the deposition is carried out with relative rotation and axial reciprocation of the support and the vapor source whereby successive layers are built up by helicoidal passes until the deposit has reached the necessary radial thickness. With ICVD and a supporting tube of silica glass, the support may be collapsed together with the vitrifiable tubular structure to form an external sheath on the resulting fiber. With OCVD, on the other hand, the supporting mandrel is removed before the collapsing step.
The high temperatures required in these processes give rise to technical problems and necessitate the use of complex equipment including automatic-control systems designed to maintain the operating temperature in a well-defined range. Because of the low density of the vapors, the deposition is relatively slow since the rate of reactant feed must be limited in order to prevent the entrainment of bubbles of unreacted halide into the structure of the preform which, after vitrification, would impair the optical qualities of the fiber. These optical qualities would also be adversely affected by occlusions of metallic impurities or water molecules.